To obtain hydrocarbons, a drilling tool is driven into the ground surface to create a borehole through which the hydrocarbons are extracted. Typically, a drill string is suspended within the borehole. The drill string has a drill bit at a lower end of the drill string. The drill string extends from the surface to the drill bit. The drill string has a bottom hole assembly (BHA) located proximate to the drill bit.
Drilling operations typically require monitoring to determine the trajectory of the borehole. Measurements of drilling conditions, such as, for example, drift of the drill bit, inclination and azimuth, may be necessary for determination of the trajectory of the borehole, especially for directional drilling. As a further example, the measurements of drilling conditions may be information regarding the borehore and/or a formation surrounding the borehole. The BHA may have tools that may generate and/or may obtain the measurements. The measurements may be used to predict downhole conditions and make decisions concerning drilling operations. Such decisions may involve well planning, well targeting, well completions, operating levels, production rates and other operations and/or conditions. Moreover, the measurements are typically used to determine when to drill new wells, re-complete existing wells or alter wellbore production.
The measurements may be associated with a time that the measurements of drilling conditions are obtained. Typically, the tools have an internal timing mechanism synchronized with a computer located at the surface before the tools are used in the borehole. During use in the borehole, the tools obtain the measurements and associate the measurements with corresponding time data provided by the internal timing mechanism. The computer periodically calculates and records depths of the drill bit and associates a time with each depth of the drill bit. Thus, when the tools are retrieved from the borehole, the tools may transfer the measurements and the corresponding time data to the computer. The computer may use the time data to associate the measurements with corresponding depths of the drill bit. The computer may generate a log of the measurements as a function of the depth of the drill bit.
The above-described method of associating the depth of the drill bit with the measurements from tools retrieved from the borehole was acceptable for drilling that had relatively low rates of penetration, such as, for example, one hundred feet per hour or less. However, modern drilling operations may achieve rates of penetration over four hundred feet per-hour, such as, for example, approximately one thousand feet per hour, requiring analysis of the measurements while the tool is located in the borehole. In addition, the internal timing mechanism of the tool may experience drift relative to the computer located at the surface such that the time indicated by the internal timing mechanism may not match the time indicated by the computer. Drift of the internal timing mechanism varies for each tool and depends on time of use and temperature encountered. The drift causes inaccuracies in the log of the measurements as a function of the depth of the drill bit.
Technology for transmitting information within a borehole, known as telemetry technology, is used to transmit the measurements from the tools of the BHA to the surface for analysis while the tool is located in the borehole. However, the transmission of the measurements by a relatively slower telemetry technology may be hindered by the inclusion of the time data. Moreover, the telemetry technology may not have the capability to transmit the time data with the measurements. Instead of using time data transmitted from the tools, the computer located at the surface may calculate an estimated time to associate with the measurements received by the computer.
However, the estimated time is typically based on several assumptions that may render the estimated time inaccurate. For example, the estimated time may be based on assumptions regarding a rate of data acquisition for the tool, a data processing time for the tool, a data acquisition time for the telemetry system, a data processing time for the telemetry system, a data transmission time for the telemetry system, a data processing time for the computer and/or the like. These assumptions may vary in actual value and/or may be difficult to calculate. For example, the type of telemetry system used, an amount of data transmitted by the telemetry system and a depth of the tool from which the measurements are transmitted may cause a variance in the transmission time for the telemetry system that may render the estimated time inaccurate.